Secondary batteries which show easy applicability to different product groups and have electrical properties, such as high energy density, have been applied generally to electric vehicles (EV) or hybrid vehicles (HV) driven by an electrical driving source as well as to portable instruments.
Such secondary batteries not only have a primary advantage of reducing use of fossil fuel significantly but also generate no byproduct resulting from use of energy, and thus have been given many attentions as novel energy sources capable of providing ecofriendly characteristics and improving energy efficiency.
FIG. 1 is an exploded perspective view illustrating the structure of the conventional pouch-type secondary battery schematically. Referring to FIG. 1, the conventional pouch-type secondary battery includes an electrode assembly 10 and a pouch casing 20 as a fundamental structure.
Herein, the electrode assembly 10 includes a positive electrode plate, a negative electrode plate and a separator interposed between the positive electrode plate and the negative electrode plate so that they may be insulated electrically from each other. In addition, the electrode assembly 10 is provided with a positive electrode tab extended from the positive electrode plate and a negative electrode tab extended from the negative electrode plate.
The positive electrode tab and the negative electrode tab may be bound to a positive electrode lead 11 and a negative electrode lead 12, respectively, through resistance welding, ultrasonic welding, laser welding, or the like. Such electrode leads are exposed to the outside of the pouch casing to carry out a function of connecting the secondary battery with an external applicable instrument electrically, as electrodes of the secondary battery.
The electrode assembly 10 is introduced to the pouch casing 20 together with an electrolyte.
The pouch casing 20 may be divided into an upper pouch 21 and a lower pouch 22, and may also be referred to as a single cap or double cap depending on the location of a portion where the electrode assembly 10 is received.
Such a pouch casing 20 may include aluminum foil inserted therein in order to protect an electrolyte introduced into the pouch casing and the electrode assembly 10, to supplement the electrochemical properties of a battery cell and to improve heat radiation property. Herein, the aluminum foil may have an insulation layer formed on the outside thereof, and the insulation layer may be coated with an insulation material, such as polyethylene terephthalate (PET) resin or nylon resin, to ensure insulation between the battery cell and the outside.
The pouch casing 20 may be bonded or adhered at the outer circumferential portion thereof through hot fusion, or the like, during a sealing process. To accomplish this, the bottom surface of the upper pouch 21 and the top surface of the lower pouch 22 may have an adhesive layer including casted polypropylene (PP) or polypropylene (PP) for the purpose of adhesion with each other. Such an adhesive layer functions to perform adhesion of the pouch casing 20 and serves as an insulation layer capable of preventing electric contact between the aluminum layer and the electrolyte introduced into the pouch casing 20.
FIG. 2 is an enlarged sectional view illustrating portion A and portion B of FIG. 1. Referring to FIG. 2, the upper pouch 21 has a predetermined layered structure having an insulation layer 25, an aluminum layer 24 and an adhesive layer 23 successively, and the lower pouch 22 includes an adhesive layer 23, an aluminum layer 24 and an insulation layer 25.
To carry out sealing of the pouch casing 20, heat and pressure may be applied to the bottom adhesive layer of the upper pouch 21 and the top adhesive layer 23 of the lower pouch 22.
FIG. 3 to FIG. 5 are schematic views illustrating a sealing portion pushed in a direction toward the inside of a cell, when a pouch casing is sealed according to the related art.
Referring to FIG. 3 to FIG. 5, the adhesive layer present at the sealing portion melts and flows due to the heat generated during sealing to form a temporary attachment region C, where the sealing portion is pushed in a direction toward the inside of the cell based on the parallel bonded end of the sealing portion. Such a temporary attachment region has a ball shape locally and the ball-shaped temporary attachment region is vulnerable to insulation and high-temperature durability. Currently, there is no separate system for controlling the shape of such a temporary attachment region.